1. Field of the Invention
The present invention relates to a photo-sensitive image screen provided with an electrode for applying a bias voltage, which screen is adapted for producing an electrostatic latent image of an original to be copied and regulating charged particles which pass through the screen in accordance with the latent image in copying operation.
2. Description of the Prior Art
The photo-sensitive screen of multi-layer structure of the above-mentioned type has been already known, as is disclosed in U.S. Pat. No. 3,713,734, for example. Referring to FIG. 1 of the accompanying drawings which shows schematically and fragmentally a section of a hitherto known photo-sensitive image screen, it is composed of an electrically conductive substrate 2 having fine apertures 1 formed in a uniform distribution over the whole area thereof, an insulating layer 3 formed on one surface of the substrate 2, an electrically conductive layer 4 formed over the insulating layer 3 and adapted to be applied with a bias voltage, and a photo-conductive insulating layer 5 formed on the other surface of the substrate 2.
The photo-sensitive image screen of the structure described above is used for producing an electostatic latent image of an original to be copied in a manner described below.
In the first place, the photo-sensitive screen is subjected to a primary corona discharge in a dark place, whereby the photo-conductive insulation layer 5 is electrically charged uniformly. Subsequently, the image of the original to be copied is exposed to the photo-conductive insulation layer 5 and is electrically charged uniformly. Subsequently, the image of the original to be copied is exposed to the photo-conductive insulation layer 5 thereby to form a primary electrostatic latent image of the original. Next, an insulative recording material such as paper is juxtaposed adjacent to the photo-conductive insulation layer 5 with a very small gap being maintained therebetween. In this state, a secondary discharge is effected from the opposite side i.e. from the electrically conductive layer 4, whereby charged particle beams are projected to the recording material through the apertured photo-sensitive screen. When a bias voltage is applied to the electrically conductive layer 4, there will be produced electric fields in the apertures 1 which have intensities different from one another in dependence on the quantity of electric charge stored in the peripheral portions of the respective apertures in correspondence to the primary electro-static latent image. These electric fields will serve then as accelerating or suppressing field for the charged particle beams passing through the apertures 1. As the result, the insulative recording material is subjected to the exposure of the charged particle beams having intensities corresponding to the primary electrostatic latent image, resulting in production of a secondary electrostatic latent image corresponding to the original image on the recording material. The secondary latent image thus obtained can be visualized through appropriate dry-or wet-type developing process. In this connection, it will be noted that the secondary electrostatic latent image corresponding to the negative or dia-positive of the original image can be produced with a desired polarity of electric charge by correspondingly selecting the polarity and intensity of the charged particle beam and/or the polarity and magnitude of the biasing voltage.
Heretofore, the substrate 2 of the photo-sensitive screen has been constituted by a net of fine mesh woven of fine wires having a diameter on the order of 20 to 80 microns, which wire is made of a metal such as iron, nickel, chromium, copper, zinc, aluminium or the like or an alloy such as stainless steel, brass or the like. Alternatively, the substrate 2 can be implemented as a plated mesh of 20 to 100 microns thick formed by electrolytic deposition from an aqueous solution containing salts of metals described above through a well known electroforming process. Further, the substrate 2 may be formed through a photo-etching technique in which a plate of metal or alloy described above and having a thickness on the order of 20 microns to 100 microns is formed with the fine apertures over the whole surface thereof. The density of the mesh lines of the substrate 2 will vary in dependence on the resolving power, tone graduation or the like factors. However, the line density is usually selected in the range of 50 to 300 lines/inch.
The insulating layer 3 formed on the one surface of the substrate 2 is usually made of an electrically insulating material exhibiting a high electric resistance. For example, synthetic resins such as silicone resin, alkyd resin, epoxy resin, polyester resin, acrylic resin, vinyl resin or the like may be used. The insulating layer 3 may be formed by applying resin material solved in a suitable solvent through spraying or application by a brush on the conductive substrate 2 in such manner that the apertures remain opened. Alternatively, the insulating layer 3 may be formed of an organic compound such as polyparaxylene or an inorganic compound such as silicon dioxide or the like through vacuum evaporation. The thickness of the insulating layer 3 will vary in dependence on the physical properties of the material as used and practically lies in the range of 5 to 100 microns.
The electrically conductive layer 4 for applying the bias voltage may be formed of a metal such as gold, silver, aluminum, nickel or the like through vacuum evaporation or alternatively formed of an electrically conductive paint containing finely pulverized powder of the metal described above dispersed in a resin adhesive through brushing or spraying application, while preventing invasion of the paint material into the apertures 1. The thickness of the conductive layer is usually on the order of less than 1 micron.
As hereinbefore described, the essential function of the conductive layer 4 is to produce electric fields in the apertures 1 for accelerating or blocking the charged particles which pass through the photo-sensitive screen and to the recording material. To this end, the conductive layer 4 has to be applied with the bias potential uniformly over the whole area thereof. The application of such bias voltage may be realized through a lead wire conductor connected to an end of the conductive layer 4 after the formation thereof by using a heat-fusible solder alloy or an electrically conductive adhesive paint. It has been however found that the connection of the lead wire through soldering under heat is accompanied with drawbacks. For example, because of the conductive layer being very thin it is difficult to obtain a satisfactory adhesion or bond. Besides, there may arise destruction of the insulating layer as well as flow of the fused solder material into the adjacent apertures 1. On the other hand, in the case of using electrically conductive paint, poor adhesion or bonding will often result due to the fact that the conductive layer itself is of mesh structure, although the destruction of the insulating layer can be prevented. Further, the liquid paint will be likely to flow in the apertures 1 formed in the substrate 2, possibly involving formation of short-circuits to the substrate. The bonding of the lead wire with the electrically conductive paint is thus impractical. It is also conceivable that the electrode or contact for applying the bias voltage to the electrically conductive layer is constituted by an electrically conductive leaf spring secured to a frame for supporting the photo-sensitive screen in the tensioned state instead of securing directly the lead wire to the electrically conductive layer 4. For example, a resilient plate of phosphoric bronze fixedly mounted on the frame is brought into contact with the electrically conductive layer 4 under pressure to be used as the electrode for applying the bias voltage thereto. However, in this case, there may arise the possibility of an unsatisfactory electrical contact between the terminal electrode and the electrically conductive layer 4 as well as physical destruction thereof due to friction between the contacting surfaces.